Automatic darkening filters commonly have a switchable filter that automatically changes from a light-transmission-state to a dark-transmission-state in response to incident light. The switching is generally achieved through use of a photodetector that is located on, or as part of, personal protective equipment. The photodetector recognizes the presence of the incident light-to-be-filtered, and an electronic module generates a control voltage that, when applied to the switchable filter, causes the filter to change from the light-transmission-state to the dark-transmission-state.
Automatic light filters have been designed which contain liquid-crystal cells located between polarizing films. U.S. Pat. No. 4,240,709 to Hörnell describes a switchable filter that has a single-twisted, nematic, liquid-crystal cell sandwiched between a pair of mutually crossed polarizers. The liquid-crystal cells are generally flat, optically-transparent, glass substrates that include transparent electrode and alignment layers. The liquid-crystal molecules orientate themselves in a particular direction when a voltage is applied across the liquid-crystal cell under the control of an electronic module. Many commercially available products use this kind of switchable filter.
The use of an automatic-darkening filter in a protective shield gives significant ergonomic benefits. Previously welders, for example, had to “nod” their welding shield down when they struck the welding arc to ensure that their eyes were protected from the torch light. Automatic welding filters eliminated this action since the welding shield could be continuously placed in the down position. As a result, weld pattern quality has been generally improved because more accurate electrode placement can be achieved. Productivity improvements also have been noted since grinding and rework have been correspondingly reduced.
Existing flat-glass automatic darkening filters can, however, add considerable weight to the final product (such as welding shield), which in turn, can create stress and tension in the user's neck and shoulders. The rectangular configuration of the typical glass sandwich construction also tends to limit the wearer's field of view. Known welding filters have been generally limited to rectangular constructions because of difficulties in scribing and breaking the rigid glass substrates.